Electroless plating (or electroless deposition) of copper and other metals has received increasing interest in recent years. This interest is due in part because of the relatively low cost of electroless processes compared to other (e.g., vacuum) deposition techniques, and because of generally surface-controlled, selective, conformal deposition properties of electroless processes. Electroless deposition has a number of potential applications, such as repair of marginal seed layers for copper damascene electroplating, creation of seed layers and barrier layers directly on dielectrics that can be plated, and selective deposition of barrier and electromigration capping layers onto damascene metal (e.g., cobalt and cobalt alloys on copper).
Conventional electroless metal deposition is conducted in a system containing one or multiple open baths containing plating solution. In a typical operation, a wafer holder immerses a substrate wafer face down in the plating solution during plating operations. The plating solution is exposed to ambient air, especially when the substrate wafer is being moved and the wafer holder does not cover the plating bath surface. Thus, an open bath system has disadvantages. For example, during the metal deposition step, ambient oxygen is readily dissolved in the solution, and the dissolved oxygen can interfere with the desired metal deposition (e.g., by slowing or preventing metal deposition). Electroless plating operations are performed at elevated temperatures in a range of 40° C. to 90° C., typically in a range of about 50° C. to 80° C. At these temperatures, the plating solution components have a tendency to evaporate. The tendency of water and volatile components to evaporate is exacerbated by the need to ventilate the gaseous spaces over a plating bath, especially to remove explosive or toxic fumes inherent to the electroless solution (e.g., ammonia gas) or created by spontaneous decomposition of its components (e.g., dimethylamine, hydrogen). The heating load caused by evaporation substantially increases the size and costs of a heater required to maintain plating bath temperature. Condensation of evaporated bath constituents on plating-cell walls and on the wafer holder are a source of backside contamination. Subsequent crystallization of those same condensates causes other contamination problems. Maintaining bath concentration, therefore, requires complicated and expensive monitoring and control techniques. See, for example, U.S. Pat. No. 6,537,416, issued Mar. 25, 2003 to Mayer et al., and U.S. Pat. No. 6,713,122, issued Mar. 30, 2004, to Mayer et al., which are hereby incorporated by reference. A conventional electroless plating bath typically can have a bath volume of 20 liters or more. Typical bath turnover rates required to avoid plate-out and composition drift are 6 hours to 10 hours. Assuming a processing rate of 20 wafers per hour, approximately 160 wafers can be processed with 20 liters.
A problem of both face-down and face-up plating configurations is hydrogen-bubble entrapment on the plating surface and resulting defects. Hydrogen gas is created as a byproduct of almost all known electroless plating-solution reducing agents. A byproduct of most electroless plating oxidation half-reactions (i.e., the oxidation of the reducing agent) and of the self-degradation of the reducing agents is dissolved molecular hydrogen (H2). As these reactions continue (i.e., plating reactions and bath-aging), the amount of hydrogen increases until the solution becomes saturated and eventually supersaturated with dissolved hydrogen. When this occurs, the formation of hydrogen gas (bubbles) is spontaneous, and occurs most readily on solid interfaces (e.g., vessel walls, wafer surfaces). Areas in which bubbles are attached to the wafer are not plated, creating defects. Therefore, it is advantageous to utilize designs that minimize the propensity for hydrogen formation, or minimize the effective bath age. United States Patent Application Publication No. 2003/0141018, by Stevens et al., published Jul. 31, 2003, teaches an electroless deposition apparatus in which a substrate support holds a substrate wafer in a face-up orientation to reduce bubble-formation and an evaporation shield is positioned over the substrate to form a gap that is filled with liquid plating solution.
Solution pH influences the reaction rate of the electroless plating process. It is often useful to utilize an alkaline pH-adjuster, for example, lithium-, sodium-, or potassium-hydroxide, but preferably ammonium- or tetramethylammonium hydroxide (“TMAH”) to maintain or adjust the pH. Alkali metal pH-adjusters are inexpensive, but are often unsuitable for semiconductor applications because of their rapid diffusion into and poisoning of various device materials. Ammonium hydroxide is also inexpensive and does not generally degrade device performance, but it is volatile. Therefore, the maintenance of ammonium hydroxide concentration in a plating bath is problematic. TMAH and other analogous organic cation hydroxides do not suffer from either of these problems, but are significantly more expensive. The constituents of a semiconductor electroless plating solution, particularly the reducing agents and TMAH, can be expensive, leading to bath costs in a range of $25/liter to $100/liter. Therefore, one would like to use lower cost materials without the negative impacts. Also, the waste treatment of electroless plating solutions is complicated and expensive. A waste treatment process generally involves forced decomposition of the reducing agents, accompanied by hydrogen gas stripping and dilution. A small amount of dissolved reducing agent can spontaneously breakdown to create a large volume of hydrogen gas in a storage container (an explosive hazard), so the removal of reducing agents must be driven to completion. A plating solution must also be stripped of metal. The cost of such plating solution post-processing (including capital equipment costs) is typically in a range of $5/liter to $10/liter. Inefficient use of the plating solution, therefore, increases the cost of plating operations significantly.
Electroless plating solutions are also often inherently unstable. The instability manifests itself in auto-degradation of bath constituents and in the “plating-out” of bath metal as fine metallic particulate in the bulk solution and onto processing equipment walls, filters, and other system components. The presence of plate-out particles also increases the number of defects in the workpieces and diminishes process yield. Generally, the instability of plating solutions increases with reducing agent concentration and with temperature, and decreases with the addition of bath “stabilizers” (e.g., oxygen, chlorine, lead, tin, cadmium, selenium, tellurium). In opposition to this trend, the initiation of electroless plating of a particular metal onto a substrate and the plating deposition rate are also proportional to reducing agent concentration and temperature, and decrease with the addition of bath stabilizers. Thus, plating-solution instability and electroless plating rate and nucleation are inherently linked in a non-advantageous manner.
Spray techniques have been suggested for electroless plating. See, for example, U.S. Pat. No. 6,065,424, issued May 23, 2000 to Shacham-Diamand et al. In such techniques, reacting plating solution is applied to a wafer surface as a spray or mist. Typically, the wafer is rotating under the spray or mist, and liquid solution is spun radially outwards. Under such conditions, it is difficult to maintain a sufficiently high and uniform reaction temperature because of the simultaneous cooling of the hot fluid by evaporation of the solvent (e.g., water). Alternatively, heating the backside of the wafer by a heated chuck is possible. Nevertheless, this requires a relatively massive element with sufficient heat capacity to maintain a globally uniform temperature over a standard 200 millimeter (mm) or 300 mm wafer. Also, the face-up base of the heating element/chuck is susceptible to chemical contamination and transfer of that contamination to the wafer backside. Furthermore, backside heating does not solve the problem of non-uniform evaporation and cooling of the bath solvent.
A wafer chuck should be capable of spinning at high revolutions per minute (rpm) to enable spin-drying. Splashing of liquid against apparatus walls and misting back onto the product surface can cause contamination of the apparatus and defects on the workpiece. Evaporation and misting of plating solution into the plating space results in substantial loss of the plating solution, and unwanted formation of volatile hazardous chemicals in the effluent.
Wet processing of isolated conductive-metal circuits connected to transistor elements in the presence of light and electrolyte often encounters a number of processing challenges. One problem is the creation of a photo-induced power source when p-n junctions in the base-circuit transistors are exposed to light. Another problem is the completion of a corrosion circuit on the surface being processed between the exposed isolated metal lines and a processing electrolytic solution. The energy of the light photons is converted to electrical energy, creating a reverse bias potential and a corrosion circuit.
Thus, liquid chemical reaction techniques, for example, immersion bath and spraying techniques, typically encounter problems such as: difficult or unsuitable control of reaction and process conditions; inability rapidly or dynamically to vary various operating conditions; inability to handle unstable reaction mixtures; accumulation of reaction byproducts; inefficient use of expensive liquid solutions; frequent wafer-handling between process steps; high capital cost of equipment for multi-step processes; and excessive use of valuable clean-room floor space.